A rotationally resolved two-dimensional photoelectronspectroscopic study of vibrational autoionization in H2

Abstract

Two-dimensional photoelectron spectroscopy, in whichphotoelectron yield is measured as a function of both photonand electron energy, has been used to investigate rotationaltransitions associated with vibrational autoionization inmolecular hydrogen, following excitation by synchrotron radiation. The energy resolution achieved in this study wassufficient to separate individual rotational transitions andhence determine the change in rotational quantum number betweenthe initial, neutral and final, ion states. Rather thanconcentrate on the rotational decay routes of particular autoionizing Rydberg states, advantage has been taken of thecomprehensive nature of the two-dimensional photoelectron spectra (2DPES) toperform a more general type of analysis. Constant transitionenergy spectra (CTES) were extracted from the 2DPEScorresponding to specific rotational transitions J''-J = 1-1,2-2, 3-3 and ΔJ = + 2, between the initial, neutral andfinal, ionic states. The ΔJ = 0 spectra proved to bebroadly similar, once an allowance was made for the differentamounts of rotational energy involved, and the comparison highlighted areas in which differences occurred. In many caseslow-n high-υ interlopers, Rydberg statesconverging on higher vibrational ionic thresholds than membersof the main series, were found in these spectral regions,suggesting that these Rydberg states have a significant effecton the rotational spectrum. Analysis of the ΔJ = + 2 CTESrevealed a tendency for these low-n high-υ interlopers to feature strongly in these spectra and this hasbeen tentatively associated with the relatively long lifetimeof these states against autoionization.